This invention relates to a control device of a vehicle mounted, charging generator driven by, for example, an internal combustion engine thereof for regulating an output voltage of the motor generator to a predetermined value and, particularly, to a protection circuit for use in such control device for protecting an output transistor of the control device when an excess current flows due to, for example, a short-circuit of a field winding of the generator.
FIG. 1 shows an example of a conventional voltage regulator of a vehicle-mounted generator in which a three-phase a.c. generator 1 includes a three-phase star wired armature winding 101 and a field winding 102. A full wave rectifier 2 for rectifying an a.c. output of the generator 1 includes a first d.c. output terminal 201, a second d.c. output terminal 202 and a grounding terminal 203. voltage regulator 3 controls an output voltage of the generator 1 to a first predetermined value by controlling a field current flowing through the field winding 102 and includes a surge absorbing diode 301 connected across the field winding 102, output transistors 302 and 303 for on-off controlling the field current, a resistor 304 constituting base circuits of the transistors 302 and 303, a transistor 305 for on-off controlling the output transistors 302 and 303, a Zener diode 306 responsive to the output voltage of the generator 1 for turning-on when the output voltage becomes equal to or higher than the first predetermined voltage, resistors 307 and 308 connected in series with each other and constituting a voltage driver circuit for the output voltage of the generator 1, a limiting transistor 309 connected in parallel to the control transistor 305 for on-off controlling the transistors 302 and 303, level shift diodes 310 and 311 connected in series with a base terminal of the transistor 309, a first detection diode 312 for detecting a collector voltage of the control transistor 305, a second detection diode 313 for detecting a collector voltages of the output transistors 302 and 303, a base resistor 314 disposed in the base circuit of the limiting transistor 309, a battery 4, a key switch 5 and a charge indicator lamp 6.
In operation, when there is no short-circuit between phases of the field winding 102 and the key switch 5 is closed to input the engine, a base current flows from the battery 4 through the key switch 5, the charge indicator lamp 6 and the base resistor 304 of the output transistor 303 to turn the latter transistor on and thus the output transistor 302 is turned on. Upon the conduction of the transistor 302, a field current flows from the battery 4 through the key switch 5, the charge indicator lamp 6, the field winding 102 and the transistor 302 and thus a field magnetomotive force is generated. At this time, a collector-emitter voltage of the output transistor 302 is lower than a sum of a base-emitter voltage of the limiting transistor 309 and a forward voltage drop of the level shift diodes 310 and 311 and thus the transistor 309 is in an off state.
Then when the engine is started to drive generator 1 and an output voltage of the generator 1 exceeds the first predetermined value of the voltage regulator 3, the Zener diode 306 and the control transistor 305 are turned-on and the output transistors 302 and 303 are turned-off. Therefore, the output voltage of the generator 1 is lowered. Since, at this time, the control transistor 305 is conductive, a collector-emitter voltage of the latter becomes lower than a sum of the base-emitter voltage of the control transistor 309 and the forward voltage drop of the diodes 310 and 311 and thus the limiting transistor 309 is kept non-conductive.
When the output voltage of the generator is lowered to the first predetermined level, the output transistors 302 and 303 are turned on again to energize the field winding 102 to thereby increase the output voltage of the generator. The output voltage of the generator 1 can be regulated at the first predetermined level upon repetitions of the above mentioned operations.
On the other hand, if the field winding 102 is short-circuited or there is an inter-phase short-circuit, when the key switch 5 is closed while the engine is in the rest state, the collector current of the output transistor 302 is sharply increased. However, since the base current of the transistor 303 is limited by the resistor 304, amplification is not sufficient and the voltage between the collector and the emitter of the transistor 302 becomes equal to or above a sum of the base-emitter voltage of the limiting transistor 309 and the forward voltage drop of the diodes 310 and 311. Since the engine is in the rest state at this time and there is no output generated by the generator 1, the Zener diode 306 and the control transistor 305 are non-conductive. Therefore, a current flows through the resistor 314, the diodes 310 and 311 and the base-emitter of the limiting transistor 309 to turn the latter on. Thus, the output transistors 302 and 303 are cut off, so that a breakdown of the output transistor 302 and 303 due to very large collector currents can be prevented.
In the conventional device shown in FIG. 1, the collector current of the output transistor 302 and hence the collector current of the output transistor 303 necessary to turn-on the limiting transistor 309 is usually set at a very large value compared with the collector current in the normal conducting state, as shown in FIG. 2. Therefore, there is no large increase of current in a case of inter-phase short-circuit of the field winding 102. Further, in a conventional system where the field current is supplied from an output terminal of a second rectifier, the output voltage of the generator 1 is lowered in a case of rare short-circuit. Therefore there is no large increase of the collector currents of the transistors 302 and 303 and the collector potential of the transistor 302 and 303 does not increase to a value at which the limiting transistor 309 is turned-on. Therefore, the transistors 302 and 303 may be broken.
FIG. 3 shows another example of the conventional device which includes, in addition to a circuit construction which is similar functionally to that in FIG. 1, a drive switching circuit 8. The latter circuit 8 includes first drive Darlington transistor 801 and 802 for driving the charge indicating lamp 6, a resistor 803 constituting a base circuit of the first drive transistors second drive Darlington transistors 804 and 805 which are turned-on when the first drive transistors 801 and 802 feed a load 7, a resistor 806 limiting base currents of the second transistors 804 and 805, a switching transistor 807 which becomes conductive at the voltage between the first and second predetermined voltage showing the power generating condition, a resistor 803 for limiting a current flow to the resistor 806 during the time transistor 807 is in an off state, a Zener diode 809 connected to a base of the transistor 807 and adapted to be turned on when the voltage of the second rectifier output terminal 202 is equal to or larger than the second predetermined value and a resistor 810 for limiting a current flow through the Zener diode when the latter is in an on state.
The operation of the circuit constituted with the components 1, 2 and 3 of the device in FIG. 3 is similar to that of the circuit in FIG. 1 except that the circuit component 3 in FIG. 3 does not include the limiting transistor 309 and the other elements associated therewith. Therefore it may be unnecessary to explain an operation thereof and it may be enough to explain an operation of the drive switching circuit 8.
The voltage at the second rectifying output terminal 202 is below the second predetermined value during the starting stage of the generator 1. Therefore, the Zener diode 809 and the switching transistor 807 are in the off state. Thus, the second drive transistors 804 and 805 are in the off state. On the other hand, the first drive transistors 801 and 802 are supplied with base currents through the resistor 803 and are in the on-state, causing the charge indicator lamp 6 to be lit. Then, when the output voltage of the generator 1 is regulated to the first predetermined value, the Zener diode 809 and the switching transistor 807 become conductive and the first drive transistors 801 and 802 are turned-off. The second drive transistors 804 and 805 are supplied with base currents through the resistor 806 and thus turned-on. Therefore, the indicator lamp is extinguished and the load 7 is fed through the transistors 804 and 805.
However, when the load 7 is short-circuited while it is fed with a current through the transistors 804 and 805 with the output voltage of the generator 1 being regulated to the first predetermined value, a large current flows through the second drive transistors leading to the breakdown thereof.